Various types of dot matrix line printers have been proposed and are in use. In general, dot matrix line printers include a print head comprised of a plurality of dot printing mechanisms, each including a dot forming element. The dot forming elements are located along a line that lies orthogonal to the direction of paper movement through the printer. Since paper movement is normally vertical, the dot forming elements usually lie along a horizontal line. Located on the side of the paper remote from the dot forming elements is a platen; and, located between the dot forming elements and the paper is a ribbon. During printing, the dot forming elements are actuated to create one or more dots along the print line defined by the dot forming elements. The paper is incremented forwardly after each dot row is printed. A series of dot rows creates a row of characters.
In general, dot matrix line printers fall into two categories. In the first category are dot matrix line printers wherein only the dot forming elements are shuttled. In the second category, are dot matrix line printers wherein the entire print head, e.g., the actuating mechanism, as well as the dot forming elements, is shuttled. Regardless of type, the portion of the dot printing mechanism to be shuttled is mounted on a carriage and the carriage is moved back and forth (e.g., shuttled) by a shuttling mechanism. The present invention is useful with both categories of dot matrix line printers. More specifically, while the invention was developed for use in connection with a dot matrix line printer wherein the entire print head is shuttled, the invention can also be utilized with dot matrix line printers wherein only the dot forming elements are shuttled.
In order to overcome the speed and other limitations of shuttle mechanisms that include stepper motors or constant speed AC or DC motors, proposals to utilize linear motors to shuttle the print head of line printers (both character and dot matrix line printers) have been proposed. For example, U.S. Pat. No. 3,911,814 discloses a character line printer wherein a hammer bank is supported by flexures. The hammer bank is oscillated between two selected positions by a linear motor. U.S. Pat. No. 4,180,766 discloses the print head of a dot matrix line printer supported by linear bearings and moved along a linear path-of-travel by a linear motor. A commercially available dot matrix line printer including a print head supported by flexures and shuttled by a linear motor is the Model 2608A line printer produced by the Hewlett Packard Company, 1501 Page Mill Road, Palo Alto, Calif. 94304. A further disclosure of a dot matrix line printer including a flexure supported print head oscillated by a linear motor is contained in U.S. patent application Ser. No. 373,802 filed May 3, 1982, by Gordon C. Whitaker and James A. Stafford and assigned to the assignee of the present application, Mannesmann Tally Corporation.
In summary, in the past, proposals have been made to utilize linear motors to shuttle the print heads of line printers. The line printers have included both character and dot matrix line printers and the print heads have been supported by both flexures and other mechanisms, such as linear bearings.
In the past, dot matrix line printers including flexure supported print heads and linear motor print head shuttling mechanisms have only been operable at medium to low speeds (300 lines per minute or less). Higher speed operation has been unsatisfactory due to vibration problems. One vibration problem is associated with the fact that the movable element of a linear motor follows a linear path while a flexure supported print head does not follow a linear path-of-travel. Rather, the path-of-travel of a flexure supported print head is arcuate. The different paths of travel create vibrations at the interface between the print head and the movable element of the linear motor. A second vibration problem is caused by the fact that the movable element of the linear motor changes configuration due to the electromagnetic stress created when the linear motor is energized by a suitable AC power source. In the case where the movable element is coil wound on or in the shape of a bobbin, the bobbin is distorted as the driving electromagnetic field changes. Specifically, the ends of the bobbin diaphragm inwardly and outwardly. Further, the bobbin is alternatively stretched and compressed as the electromagnetic field produced by the current through the bobbin coil changes. These changes in configuration are coupled to and vibrate the print head. Uncontrolled vibration of the print head makes it difficult, if not impossible, to precisely position dots. As a result, printed characters become distorted resulting in an unacceptable printed product. The present invention is directed to substantially reducing, or entirely eliminating, the effect of the just described vibration problems.